In wireless networks, sleep mode is designed to reduce the power consumption by subscriber stations in the network. While in sleep mode, a subscriber station (SS) may alternate between an availability interval (AI) and an unavailability interval (UAI). During an unavailability interval an SS may power down its radio interface(s). On the other hand, during an availability interval, the subscriber station can communicate with the base station to send and/or receive data or management traffic. Thus, a subscriber station may send and/or receive traffic while in a sleep mode. However, the amount of traffic that can be exchanged and the delay associated with it depends on the duration and frequency of the availability interval. This is because the traffic can be exchanged between a subscriber station in sleep mode and a base station only during an availability interval. This is not a problem for real time traffic that is periodic in nature. For real time traffic such as voice over internet protocol (VoIP) traffic, traffic may arrive once in every T seconds. Moreover, the amount of traffic in every T seconds may be generally constant. Therefore, a subscriber station in sleep mode can easily send and/or receive such traffic by aligning its availability intervals to the arrival time of the traffic. Moreover, as the traffic length is more or less constant, the duration of the availability interval may be selected to allow the transfer of the expected amount of traffic.
On the other hand, the support of best effort (BE) traffic in sleep mode may be an issue. Two parameters may be defined to characterize such traffic: traffic load (TL) and delay requirements (DR) of the BE traffic. Based on these two parameters, BE traffic can be broadly classified into the following four groups.
Class A: BE traffic with low TL and less stringent DR
Class B: BE traffic with low TL and stringent DR
Class C: BE traffic with high TL and less stringent DR
Class D: BE traffic with high TL and stringent DR
A subscriber station in sleep mode can send and/or receive Class A BE traffic using the AI intervals. However, the subscriber station may not be able to send and/or receive other BE traffic classes using the AI intervals. Thus, the following two issues are considerations for supporting best effort traffic with higher traffic loads and/or stringent delay requirements in sleep mode.
The latency of the BE traffic (Class B and D) should be kept as low as possible. Therefore, waiting to transmit data only during AI may not be ideal, especially when the duration of AI is small. In a scenario where the base station has to send X amount of data to a subscriber station in sleep mode, the base station waits until the next AI of the subscriber station and during the next AI the base station could send only Y (<X) amount of data allowed by the length of AI interval, and waits to transmit the remaining (X-Y) amount of data till the another AI. This increases the delay in sending the total X amount of data. Such delay can be more when the duration of UAI is long.
Uncontrolled growth of the buffered amount of BE traffic at the base station and/or the subscriber station when the subscriber station is in sleep mode should be avoided. Thus, sleep mode should support optional mechanisms for the subscriber station to send and/or receive any amount of traffic during its AI interval. Using such option, a subscriber station in sleep mode could send and/or receive traffic without terminating its sleep mode operation. As a result, the overhead associated with repeated trigger/termination of sleep mode could be reduced, and further power saving at the subscriber station could be increased. A subscriber station in sleep mode could retain the option to terminate its sleep mode to send and/or receive traffic.
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